Infrared laser absorption spectroscopy is an extremely effective tool for detecting trace gases. The demonstrated sensitivity of this technique is at the parts per billion (ppb) level. Presently, the usefulness of the laser spectroscopy approach is limited by the availability of convenient tunable sources in the spectroscopically important “fingerprint” region from 3 to 20 μm. The available options include cryogenically cooled lead salt diode lasers and coherent sources based on difference frequency generation (DFG). Sensors based upon lead salt diode lasers are typically large in size and require consumables because the diodes operate at temperatures below 90 K. DFG based sources (especially PPLN based) are shown to be very robust, but they generate inherently low IR power. DFG based sensors are suitable for many atmospheric monitoring applications. However, their spectral coverage is currently limited to wavelengths shorter than 5 μm by the optical transparency of suitable nonlinear optical crystals such as periodically poled LiNbO3.
The recent development of quantum cascade lasers with distributed feedback (QC-DFB) fabricated by band structure engineering offer an attractive option for IR absorption spectroscopy. Compared to Pb-salt diode lasers, QC-DFB lasers allow the realization of very compact narrow-linewidth mid-IR sources combining single-frequency operation and substantially higher powers (tens of mW) at mid-IR wavelengths (3.5 to 24 μm). Pulsed DFB QC lasers are semiconductor lasers able to emit mid-IR radiation at room temperature (continuous wave, or “cw”, operation requires temperatures below 150K at this time). The higher power of QC lasers permits the use of advanced detection techniques that improve S/N ratio of trace gas spectra and decrease the apparatus size. For example in Cavity Enhanced Spectroscopy (CES) and Cavity Ringdown Spectroscopy (CRDS) an effective absorption pathlength of hundreds of meters can be obtained in a laptop-size device. The large wavelength coverage available with QC lasers allows numerous molecular trace gas species to be monitored. Recent measurements with QC-DFB lasers have demonstrated the usefulness of these devices for sensitive, highly selective real time trace gas concentration measurements based on absorption spectroscopy with sensitivities of several parts per billion (See K. Namjou et al, “Sensitive absorption spectroscopy with a room-temperature distributed-feedback quantum-cascade laser”, Optics Letters, V. 23, n. 3, Feb. 1, 1998, which is hereby incorporated by reference).
Pulsed operation of QC-DFB lasers gives a unique opportunity to design a liquid nitrogen-free mid-IR spectroscopic sensor. However, specific problems are associated with this mode of operation. The peak power in pulsed mode is essentially the same as for cw operation, but the duty cycle has to be less then 1 percent to avoid an overheating of the device. Therefore, the average power is less than the power generated by the cw operation. This difference requires either more sensitive detection of average power or gated detection of peak power. Another problem is laser frequency chirping during the current pulse. This effect causes broadening of laser linewidth, limiting the spectral resolution and complicating data processing. These features associated with pulsed operation should be considered in the design of a trace gas sensor.